1. Field of the Invention
Cellulose hydrolysis, as is well known, can be achieved by a number of techniques; however, the most commonly used methods for effecting cellulose hydrolysis employ the utilization of either mineral acids or enzymes. The hardware and techniques described infra, and which comprise the instant invention, relate only to the acid hydrolysis systems.
As to such acid hydrolysis systems, they have been categorized or classified by prior art investigators as either dilute or concentrated. Referring now specifically to sulfuric acid systems, dilute sulfuric acid hydrolysis, as universally understood and practiced, is conducted with acid solutions whose effective concentration in the aqueous phase is usually less than about 10 percent. Note: unless otherwise specifically indicated, all concentrations herein are understood to be on a by weight basis. Conversely, to effect the high glucose conversion efficiencies reported in the past, and discussed infra, concentrated sulfuric acid hydrolysis systems had to be conducted with very concentrated acid solutions to ensure a minimum concentration in the aqueous phase during processing above about 70 percent. Therefore, according to the classical definitions and processing techniques adhered to in the prior art, the invention described herein falls in neither the concentrated nor the dilute category. However, for ease of understanding and convenience and further since the conditions under which the instant invention operates more closely approximates those used for concentrated acid hydrolysis, the instant process will hereinafter be described or generally categorized as a concentrated acid hydrolysis process.
Feedstocks for such cellulosic hydrolysis may be, but are not limited to, the following materials: wood; wood waste; waste paper; the cellulosic fraction of municipal solid waste; or agricultural residues such as corn stover, sugar cane bagasse, and cotton gin trash. The sugars resulting from such hydrolysis include the hexose sugars glucose, mannose, and galactose; and the pentose sugars, xylose, and arabinose.
In addition to producing such sugar products, a relatively small amount of a number of by-products originating from the other components found in the hemicellulose, or resulting from degradation of the sugars, or the extractives present in the feedstock may also be produced when cellulosic materials are hydrolyzed. For example, acetic acid, which is produced from the hydrolysis of hemicellulose, is the most prevalent by-product resulting from the hydrolysis of wood. Acetic acid is produced from the glucomannan and methylglucoronoxylan fraction of the hemicellulose. Among the by-products originating from the degradation of sugar are furfural, hydroxymethyl furfural, and levulinic acid. The feedstock extractives consist mainly of tannins, resins, and gums. Such tannins contain polyhydroxyphenols. Condensed tannins like catechin cannot be hydrolyzed. Hydrolyzable tannins consist of gallotannins, ellagitannins, and caffetannins. A gallotannin, for example, is a single molecule of glucose combined with ten molecules of gallic acid. Resins are complex structures consisting of resin acids like carboxylic acid resin alcohols, resinotannols, and resenes. Resins often occur in mixtures with volatile oils; these mixtures, an example of which is turpentine, are known as oleoresins. The gums may be classified as polysaccharidcs or salts of polysaccharides and may be pentosans or hexosans. When hydrolyzed, in addition to sugars, gums, which contain a complex organic acid nucleus, form salts primarily with calcium, magnesium, and potassium.
Although lignin is inherently associated with the lignocellulosic material feedstock, it consists of a variety of phenylpropane derivatives bound in a complex network difficult to characterize and is usually considered to be unreactive (Herman F. J. Wenzel, Chemical Technology of Wood, Academic Press, New York, 1970). Because lignin is for the most part unreactive, it is oftentimes used as a tie element in material balance calculations.
2. Description of Prior Art
Numerous prior art investigators have discovered, taught, and disclosed a plethora of methods and/or means for hydrolyzing cellulosic materials to produce both sugars, and sugar-rich products. For instance, it has been known since at least as early as 1819 that cellulose can be hydrolyzed to yield sugar. Since that time many processes have been developed for both the concentrated acid hydrolysis process and the dilute acid hydrolysis process. Following is a brief summary of some of the more significant of these prior art processes.
In 1880, a hydrolysis process based on supersaturated or fuming hydrochloric acid was patented in Germany and was known as the "Rleinau process." In addition to other problems, the corrosiveness of the processing environment made commercial application of same highly impractical.
Continued development of the Rheinau process led to a countercurrent operation that permitted much higher sugar concentrations. This improved process became known as the "Rheinau-Bergius process." In spite of the major increases in sugar concentration possible with the new processing techniques, acid consumption remained a particular problem. Approximately three parts on a weight basis of 41 percent concentrated hydrochloric acid were required for each part of wood according to Wenzel, supra.
Like the hydrochloric acid processes described supra, numerous researchers have investigated the use of sulfuric acid to effect hydrolysis of lignocellulosics. These researchers have used both dilute solutions and concentrated solutions of sulfuric acid. As noted supra, typically, dilute sulfuric acid systems utilize acid solutions containing less than 10 percent sulfuric acid, while the concentrated acid systems require greater than 70 percent sulfuric acid solutions during processing. As noted earlier, it is important to remember that processes using dilute solutions of sulfuric acid, in either a single-stage or two-stage mode, are typically operated at elevated pressures and temperatures relative to concentrated acid systems. These elevated temperature processes, as will be discussed in more detail, infra, may also cause degradation of the product sugars. Sugar degradation, especially those sugars resulting from hydrolysis of hemicellulose, caused some prior art investigators to research two-step processes in which the hemicellulose would be hydrolyzed and the resultant hydrolyzate collected before cellulose hydrolysis, see, for example, Reitter, U.S. Pat. No. 4,427,453, Jan. 24, 1984. On the other hand, concentrated sulfuric acid processes utilize solutions containing more than 70 percent sulfuric acid and are able to effect hydrolysis of cellulose at or near ambient pressures.
One of, if not, the first successful applications of dilute sulfuric acid hydrolysis technology took place at the end of the 1920s with the development of the Scholler-Tornesch process. This process, which used approximately a 0.4 percent acid solution at temperatures and pressures of around 170.degree. C. and eight atmospheres, respectively, to effect hydrolysis, employed a percolation system normally consisting of three vertical reactors.
The Scholler-Tornesch process, supra, was later brought to and tested in the United States during World War II. The results of the tests were, however, not encouraging. Due to the scarcity of ethanol during the war, the United States sponsored the development of a new dilute sulfuric acid hydrolysis process, which came to be known as the Madison process. The process, which was developed at the University of Wisconsin's Forest Products Laboratory in Madison, differed in a number of respects from the earlier Scholler-Tornesch process. The Madison process was operated as a continuous process, whereas the Scholler-Tornesch process was limited to batch operation; therefore, the Madison process could provide for much greater throughput. In addition, the Madison process operated at a much lower liquid-to-solids ratio than the Scholler-Tornesch process: 3-to-1 versus 10-to-1, respectively, which allowed for significantly increased sugar concentrations in the resulting product.
Operation of the Madison dilute sulfuric acid hydrolysis process was practiced by first adding acid, at a concentration of approximately 0.5 percent, to the reactor. The temperature of the reactor was held at 150.degree. C. for 30 minutes. At these conditions, almost all the pentosans and hexosans of the hemicellulose hydrolyzed. Subsequently, the resulting hemicellulose sugar rich solution was drawn off. More fresh acid was added to the residue remaining in the reactor and the temperature within same was slowly increased to 185.degree. C. As more hydrolyzate was removed from the reactor, more fresh acid was added. Processing time for the Madison process was approximately 3 to 3.5 hours versus 15 to 18 hours for the Scholler-Tornesch process, supra. Testing of the Madison process was stopped when the war ended since thereafter the demand for ethanol was greatly reduced. Although the plant, as constructed, consisted of five reactors, only one reactor was ever operated, and then approximately for only some six months.
Later, in the 1950s, the Tennessee Valley Authority (TVA) constructed a dilute sulfuric acid hydrolysis pilot plant which was based on the Madison process: an acid concentration of 0.5 to 0.6 percent was used at a temperature of approximately 180.degree. C. The primary difference between the Madison and TVA processes was that the TVA process employed higher pressures: 14 to 16 atmospheres versus 9 atmospheres. Total processing time was reduced from about 3 to 3.5 hours for the Madison process to about 2.5 to 3 hours in the TVA process. However, this still long processing time served to limit the commercial viability of the process by necessitating the use of very large equipment.
In the late 1970s and early 1980s, work at the University of New York lead to further development of a dilute sulfuric acid process which employed twin screw extruder technology to effect a more commercially viable dilute sulfuric acid hydrolysis process than those discussed supra by decreasing the size of the processing equipment required. The high glucose conversions observed were made possible by exposing the feedstock to a high degree of strain through intense mixing and higher temperatures for shorter times. To effectively accomplish this a commercial model twin screw extruder was used as described in Rugg et al., U.S. Pat. No. 4,316,747, Feb. 23, 1982; U.S. Pat. No. 4,316,748, Feb. 23, 1982; U.S. Pat. No. 4,363,671, Dec. 14, 1982; U.S. Pat. No. 4,368,079, Jan. 11, 1983; U.S. Pat. No. 4,390,375, Jun. 28, 1983; and, U.S. Pat. No. 4,591,386, May 27, 1986. Twin screw extruders are designed to run starved, that is, without filling all the volume in the intermeshed flights with reaction mass. These extruders can provide for acid impregnation through intense mixing. Running starved, such impregnation is accomplished with high shear and strain and not compression pressure.
As described by Rugg et al., '375 and '386, supra, column 6, lines 1-50, the twin screw extruder/reactor was used to effect the following conditions: a reaction zone temperature of 237.degree. C. (459.degree. F.), a reaction zone pressure of 400 psi, and an effective acid concentration of 1.34 percent. These conditions produced a glucose conversion of 50 percent. In order to maintain the high process pressures and temperatures within the reaction zone, Rugg et al., designed their extruder/reactor to provide for a dynamic seal upstream of the reaction zone and a small diameter orifice at the reactor's discharge point. Rugg et al., '375 and '386, supra, column 7, lines 15-16, for example, although alluding to the importance of residence time, do not describe their residence times, but rather have left it up to the reader to deduce, from the information provided in column 6, lines 25-35 and column 6, lines 25-34, respectively, and the rotational speed of the screw provided in column 6, line 15 and column 6, line 13, respectively. From the information provided in this example, it would appear that the reaction mass would have a total residence time in the reactor of between 12 and 13 seconds and a reaction zone residence time of about 7 seconds. The information provided in Rugg et al., '671, column 5, lines 39-46, and the rotational speed provided in column 5, line 29; and Rugg et al., '748, column 5, lines 37-44, and the rotational speed provided in column 5, line 26, also allows the reader to deduce a total residence time, in these two examples, of approximately 11 to 12 seconds and a reaction zone residence time of about 7 seconds. As will be discussed in more detail infra, the conversion and residence times obtained by Rugg et al., in the examples referenced supra, correlate closely with well described kinetics for dilute acid hydrolysis. For example, based on the glucose conversion and conditions described in Rugg et al., '375 and '386, supra, a residence time of about 5 to 12 seconds in the reaction zone is predicted by the kinetics.
Wherein the long reaction residence times associated with previous dilute sulfuric acid hydrolysis systems contributed to their lack of commercial viability, the short reaction residence times associated with the invention of Rugg et al., may have likewise effectively prevented its use in that physically, it is much too short to effect, with mechanical means, the conditions required for efficient glucose conversion. In addition, and as will be discussed in more detail infra, the high sugar degradation rates associated with the invention of Rugg et al., may have also played an important factor in the lack of a commercialization effort.
One of the most effective, albeit energy-demanding and complicated, processes developed to date for converting lignocellulosics to sugar was developed at the United States Department of Agriculture's National Regional Research Laboratory in Peoria, Ill. The process included the following seven separate processing steps: hemicellulose (pentosan) hydrolysis, dewatering, drying, grinding, acid mixing, acid impregnation, and cellulose hydrolysis. For a detailed description of the process see J. W. Dunning et al., Industrial and Engineering Chemistry, Vol. 37, No. 1, January 1945, "The Saccharification of Agricultural Residues," pp. 24-29; and, Dunning et al., U.S. Pat. No. 2,450,586, Oct. 5, 1948.
As taught in Dunning et al., '586, supra, column 1, line 43 through column 2, line 49, their process included no less than seven separate steps starting with using dilute sulfuric acid, 1 to 6 percent at 100.degree. C. to 121.degree. C. to convert the pentosans and hexosans contained in the hemicellulose to pentose and hexose sugars. Thereafter, the cellulose and lignin rich residue that remained was collected and mechanically dewatered. The resulting residues were therein thermally dried to produce a material containing less than 2 percent moisture. The resulting very dry material was then ground to pass a 40-mesh screen and subsequently mixed with 80 to 87 percent sulfuric acid at a temperature below 40.degree. C. The fully mixed material was then compressed under a continuously changing directional pressure above substantially 100 psi at a temperature of not more than 45.degree. C. to impregnate the feedstock with acid. The resultant mixture was then collected and hydrolyzed to produce glucose conversions of approximately 90 percent. A more detailed discussion of impregnation is provided in J. W. Dunning et al., Industrial and Engineering Chemistry, supra.
In the development of their process, Dunning et al., '586, supra, relied on the well documented dilute acid hydrolysis technology, described supra, to hydrolyze the pentosans and hexosans comprising the hemicellulose. The cellulose and lignin rich residue was then taken and mechanically dewatered to remove most of the water from the residue. Thereinafter, thermal drying with a current of hot air as described in Dunning et al., '586, column 2, lines 12-14, was required to achieve the desired moisture level of 2 percent in the resultant solid residue. Column 2 further describes that the dried solid residue was ground to pass a 40-mesh screen. The dried and ground solid residue was then mixed with 0.15 to 0.55 parts of 80 to 87 percent sulfuric acid per part of cellulosic material at temperatures below 40.degree. C. Only after all of these five steps had been carried out was the resulting solid residue subjected to continuously changing directional pressure to impregnate the acid into the cellulosic structure. As described at column 2, lines 25-35, this impregnation was effective at pressures of 100-250 psi for periods of time preferably ranging from 1 to 5 minutes at temperatures not exceeding 45.degree. C. As described in Dunning et al., Industrial and Engineering Chemistry, supra, the impregnation resulted in a compression of the feedstock to 35 percent of its original volume. As further described in Dunning et. al., '586, column 6, lines 21-24, an expeller press was used. The pressure step converted the solid residue from a free flowing powder to a stiff plastic mass. The final step, in the seven step processing scheme of Dunning et al., '586, was the conveyance of the stiff plastic mass to a container into which sufficient water was added to dilute the acid to approximately 7 to 9 percent and then the pumping of the resultant slurry, under a pressure of 5 to 45 psi through a coil hydrolyzer heated to 120 to 135.degree. C. for a period of time preferably ranging from 5 to 20 minutes.
As described in Dunning et al., Industrial and Engineering Chemistry, the hydrolyzate would, in a process to produce ethanol, be filtered to remove the unreacted cellulose and lignin, neutralized with lime to react the residual sulfuric acid; filtered again to remove the resultant gypsum; and fermented to produce ethanol.
In the nearly 50 years since the issuance of the '586 patent to Dunning et al., there has not been a single successful commercialization effort. The reasons for this may include the complexity of the process but most likely the high cost of recovery of acid associated with his process. The use of 80 to 85 percent sulfuric acid in the process taught by Dunning et al., '586, precludes the economical reconcentration and recycle of the acid using systems employing mechanical vapor recompression. The acid recovered from the hydrolysis process described by Dunning et al., '586, is less than 10 percent and must be reconcentrated back to 80 to 85 percent before reuse. It has been found that economical reconcentration of the acid can only be accomplished through the use of mechanical vapor recompression. Mechanical vapor recompression systems typically require about 30-50 BTUs to evaporate one pound of water. By comparison, a steam injection system would require about 1300-1500 BTUs to evaporate the same pound of water. As may be appreciated by those skilled in the art, this method of evaporation works best on solutions whose boiling point remains relatively constant during the evaporation process. Since the boiling point of sulfuric acid increases with increased concentration, application of mechanical vapor recompression for reconcentration beyond about 55 percent becomes problematic. From Perry et al., Chemical Engineers' Handbook, Fifth Edition, McGraw Hill-Hill Book Company, 1973 it is shown that concentrating the acid solution from 10 to 55 percent, as may be practiced in the instant invention, results in a 28.degree. C. temperature increase in the boiling point of the acid solution, from 102 to 130.degree. C. This temperature increase approximately represents the limits within which mechanical vapor recompression is economical. By comparison, concentrating an acid solution from 10 to the 80 percent level of Dunning et al., results in a temperature increase of 98.degree. C.
Because of the plethora of problems associated with sulfuric acid type hydrolysis systems, work over the last decade has all but stopped. Other research has been directed to peripherals associated with completely integrated processes. For instance, Lightsey et al., U.S. Pat. No. 5,407,817, Apr. 19, 1995, teach presegregation of municipal solid waste and pretreatment with dilute sulfuric acid to reduce heavy metal content in the recovered cellulosic component. Others have switched to studying enzymatic hydrolysis processes. Enzymatic hydrolysis could offer simple processing and high conversions. The National Renewable Energy laboratory switched its focus to enzymatic hydrolysis of cellulose in the mid 1980s. However, development of an economical enzymatic hydrolysis process has yet been realized.
Kinetic Analysis of Concentrated Versus Dilute Acid Hydrolysis Systems
In order that those skilled in the art may better understand why dilute acid hydrolysis processes, such as the one described by Rugg et al., should operate at the short residence times, discussed supra, to effect glucose conversion efficiencies of about 50 percent, the following kinetic analysis is provided. As will be demonstrated, infra, the short residence times deduced, supra, which are on the order of seconds, are predicted by the empirical kinetic relationships for dilute acid hydrolysis systems developed by other researchers. As will also be demonstrated from these same kinetics, longer residence times result in lower glucose conversion efficiencies due to the fact that the product sugar starts to degrade faster than it is formed. Therefore, in dilute acid hydrolysis systems, it is best to convert the lignocellulosic feedstock to sugar quickly and also remove the product sugar as quickly as possible to minimize sugar degradation. Because these high glucose conversions are only possible at these relatively short residence times, design of commercial dilute acid hydrolysis processing systems, capable of achieving these same results, becomes problematic.
To aid in understanding the significance of the instant invention, the dilute acid process described by Rugg et al., '747, '748, '671, '079, '375, and '386, supra, and the concentrated acid process described by Dunning et al., '586, will be relied on for use of comparison. For example, Rugg et al., '375 and '386, column 6, lines 63-64, and column 7, lines 1-2 teach that the reaction conditions within their process can vary between from 350.degree. F. (177.degree. C.) to about 545.degree. F. (285.degree. C.) at pressures of 135 to 1000 psi, respectively. From the steam tables, examples of which can be found in any of a variety of technical publications, such as Richard E. Balzhiser et al., Chemical Engineering Thermodynamics, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1972, it is noted that a saturated steam temperature of 350.degree. F. (177.degree. C.) correlates to a saturated steam pressure of 135 psi and a saturated steam temperature of 545.degree. F. (285.degree. C.) correlates to a saturated steam pressure of 1000 psi, it being understood all numbers are rounded to the nearest whole number as in Rugg et al., '747, '748, '671, '079, '375, and '386, supra.
Although teaching pressures of at least 135 psi, Rugg et al., '375 claim pressures equal or exceeding only 100 psi, which, from the steam tables, correlate to a lower reaction temperature of 328.degree. F. (164.degree. C.). It may be deduced, therefore, that Rugg et al., '375, are referring to superheated steam at 100 psi and 350.degree. F. Rugg et al., '375 and '386, column 6, lines 64-65 point out that reaction temperatures can exceed 545.degree. F. depending upon the available steam pressure, and in all their examples report reaction temperatures which correspond to at least the saturated steam temperature at the pressures described. For instance, in the example provided in Rugg et al., '375 and '386, the reaction temperature listed is actually above the corresponding saturated steam temperature at the pressure given; therefore, the steam used in this example must have been superheated. In each of the single examples provided in Rugg et al., '747, '748, '671, and '079 the reaction temperatures disclosed therein correspond to the saturated steam temperatures at the given pressures.
As is now shown, the findings of Rugg et al., '747, '748, '671, '079, '375, and '386 correlate closely to the finding of other researches who investigated the kinetics of dilute sulfuric acid hydrolysis. These researchers demonstrated that the hydrolysis of cellulose to glucose, a hexose sugar, and hemicellulose to a mixture of hexose and pentose sugars, primarily xylose, could be modeled as first-order homogeneous reactions, in which the cellulose content is expressed as potential glucose and the hemicellulose content is expressed as potential xylose (J. F. Saeman, Industrial and Engineering Chemistry, Vol. 37, "Kinetics of Wood Saccharification." pp 43-52, January 1945). The following represent simplified reaction pathways:
cellulose.fwdarw.glucose.fwdarw.hydroxymethylfurfural PA1 hemicellulose.fwdarw.xylose.fwdarw.furfural PA1 G=glucose concentration expressed as a fraction of potential glucose. PA1 k=preexponential factor, min.sup.-1 PA1 A=weight percent sulfuric acid in solution PA1 E=activation energy, cal/gm-mol PA1 R=gas constant, 1.987 cal/gm-mol .degree. K PA1 T=absolute temperature, .degree. K PA1 m,n=constants
The rate constant for the conversion of cellulose to glucose may be called K.sub.1, the rate constant for the degradation of glucose to hydroxymethyl-furfural, a degradation product, may be called K.sub.2, and the rate constant for the degradation of xylose to furfural, a degradation product, may be called K.sub.3.
The conversion of hemicellulose to xylose is considered to be instantaneous (K=.infin.). Rate constants are expressed in min.sup.-1. The rate equations for the conversion of cellulose are shown below. ##EQU1## where C=cellulose concentration expressed as a fraction of potential glucose.
These rate equations can be integrated to yield an expression which gives the fraction of potential glucose present at any given time (3) and the amount of cellulose present at any time (4). ##EQU2## Where C(0)=initial cellulose fraction.
Accounting for the acid present, the rate constants K.sub.1 and K.sub.2 can be calculated from Arrhenius' law as follows: EQU K.sub.1 =k.sub.1 A.sup.m exp(-E.sub.1 /RT) 5 EQU K.sub.2 =k.sub.2 A.sup.n exp(-E.sub.2 /RT) 6
Where
Several investigators have studied these kinetics (John F. Harris et al., "Two-Stage Dilute Sulfuric Acid Hydrolysis of Wood: An Investigations of Fundamentals," United States Department of Agriculture, Forest Products Laboratory, General Technical Report FPL-45, 1985). In 1981, a research team at Dartmouth College studied these reaction kinetics (H. E. Grethlein, "Dartmouth College: Acid Hydrolysis of Cellulosic Biomass." Alcohol Fuels Program Technical Review. U.S. Government Printing Office: 1982-576-083/201, 1981. The values given below for cellulose hydrolysis, glucose degradation, and xylose degradation were taken from the Dartmouth study.
______________________________________ Cellulose Glucose Xylose Hydrolysis Degradation Degradation ______________________________________ k.sub.1, k.sub.2, k.sub.3 (min.sup.-1) 5.33 .times. 10.sup.16 3.89 .times. 10.sup.9 8.78 .times. 10.sup.15 m, n, p 1.14 0.57 1.00 E.sub.1, E.sub.2, E.sub.3 36,955 20,988 33,560 (cal/gm-mol) ______________________________________
Rugg et al., '375 and '386, column 6, lines 5-45 teach that by operating their process at a superheated condition of 237.degree. C. (459.degree. F.) at 400 psi, (i.e., the temperature of the steam is in excess of that listed for saturated steam at that pressure in the steam table), and using an effective acid concentration of 1.34 percent sulfuric acid, it is possible to convert 130 lbs/hr of dry sawdust to 40 lbs/hr of glucose and 13 lbs/hr of hydroxymethylfurfural. According to Rugg et al., '375, the glucose conversion represents 50 percent of the available cellulose. By using the kinetic data provided, supra, rate constants can be derived as follows: EQU K.sub.1 =5.33.times.10.sup.16 (1.34.sup.1.14)exp.sup.(-36955/510.times.1.987)) K.sub.1 =10.81 min.sup.-1
and EQU K.sub.2 =3.84.times.10.sup.9 (1.34.sup.0.57)exp.sup.(-20988/510.times.1.987)) K.sub.2 =4.59 min.sup.-1
Therefore a K.sub.1 /K.sub.2 ratio of approximately 2.4 can be calculated. With K.sub.1 and K.sub.2, the amount of glucose present as a percentage of potential glucose can be calculated using equation 3, supra. It can be readily shown that the maximum glucose conversion is approximately 53 percent at 8 seconds. At times ranging from 5 to 12 seconds a glucose conversion approximately equal to or exceeding 50 percent is obtained. This conversion closely corresponds to the 50 percent conversion claimed by Rugg et al., '375 and '386, at the time deduced, supra. Isonger residence times at these conditions result in lower conversions. For example, at 30 seconds a glucose conversion of less than 17 percent is achieved, which is due to the fact that sugar is being degraded faster than it is being formed. Lower temperatures result in lower K.sub.1 /K.sub.2 ratios, which indicate lower potential conversions. Operating at higher temperatures increases potential conversion, but these higher conversions are only possible at shorter residence times. For example, at 545.degree. F. (285.degree. C.) and 1.34 percent acid, a potential glucose conversion of 69.5 percent is possible. The K.sub.1 /K.sub.2 ratio at these conditions is approximately 9.1. However, this conversion is achieved at a residence time of only 1 second. In approximately 12 seconds, essentially all the glucose has degraded. Conversely, at 350.degree. F. (177.degree. C.) and 1.34 percent acid, a maximum conversion of approximately 17.4 percent is obtained at about 6 minutes. At these conditions, the K.sub.1 /K.sub.2 ratio is approximately 0.29. Finally, by operating the process at higher acid concentrations and lower temperatures, it is also possible to achieve high glucose conversions, however, these high glucose conversions also require short reaction times. For example, using an effective acid concentration of 10 percent at a temperature of 405.degree. F. (207.degree. C.) it is possible to achieve a maximum glucose conversion of 56 percent in about 9 seconds. In this case, a glucose conversion of or exceeding 50 percent is possible at residence times ranging from about 6 to 14 seconds. Longer residence times result in lower glucose conversions. The K.sub.1 /K.sub.2 ratio for this case is approximately 2.8.
In order to minimize sugar degradation and achieve cellulose to sugar conversions greater than 50 percent in dilute acid hydrolysis systems, short reaction times are necessary. These short reaction times make commercial processes difficult to design, especially when operating with the large zone temperature differences, as much as 513.degree. F. (267.degree. C.), taught by Rugg et al., '375 and '386, column 7, lines 42-45, for example. However, as will be shown, infra, the lower temperatures associated with concentrated acid hydrolysis systems minimize sugar degradation and provide for much higher cellulose to sugar conversions.
To aid those skilled in the art in assessing the potential gains possible with concentrated acid hydrolysis systems, a kinetic analysis was conducted to determine the rate constants associated with the system of Dunning et al., supra. The ratio of the rate constants provide a tool by which it is possible to compare, in a scientific way, dilute and concentrated acid hydrolysis processes.
In 1945, the results of a study to define a continuous acid hydrolysis process which could produce glucose conversions of approximately 90 percent were published (Dunning et al., Industrial and Engineering Chemistry). Cellulose conversions of 89 percent were reported in the tests. The process used by Dunning et al. to achieve this conversion involved no less than seven separate and distinct steps. These steps included hemicellulose (pentosan) hydrolysis, mechanical dewatering, thermal drying, acid mixing, grinding, acid impregnation, and cellulose hydrolysis. Unlike the work conducted by Rugg et al., supra, Dunning et al. achieved acid impregnation using an expeller screw press, which compressed the feedstock to 35 percent of its initial volume under a pressure of 175 psi. The glucose conversions obtained in one of the tests described are given below. Unlike the dilute acid hydrolysis process, these conversions were achieved at a temperature of only 130.degree. C.
______________________________________ Time (min) Glucose Conversion @ 130.degree. C. (percent) ______________________________________ 1 20 2.5 60 6 89 10 87 15 85 20 82 ______________________________________
Although not appreciated by Dunning et al. (Industrial and Engineering Chemistry), with the data generated it is possible to perform a kinetic analysis to scientifically quantify the increased performance potential of the concentrated acid hydrolysis process over the dilute acid systems, such as the dilute acid system investigated by Rugg et al., supra.
By taking the derivative of equation 3, supra, with respect to time and setting the derivative equal to zero, the following, expression is obtained: ##EQU3## Substituting equation 7 into equation 3 and rearranging yields the following expression: ##EQU4## Since the initial cellulose concentration can be set equal to one, the expression can be simplified to yield: ##EQU5## Substituting from equation 8 yields the following expressions: ##EQU6##
By substituting the values obtained by Dunning et al. (Industrial and Engineering Chemistry), for maximum glucose conversion at six minutes, a rate constant (K.sub.2) of 0.019 min.sup.-1 is obtained. The rate constant K.sub.1 can now be derived using a root finding technique, such as the Newton-Raphson method. Again, from the Dunning et al. data, the rate constant K.sub.1 is determined to be 0.568 min.sup.-1. By comparing the rate constants and the K.sub.1 /K.sub.2 ratios for the Dunning et al. and Rugg et al. (29.9 vs. 2.4, respectively), it will be apparent to those skilled in the art that the concentrated acid hydrolysis process is indeed far superior for achieving high conversions of sugar from cellulose at reaction mass residence times far more realistic with respect to commercial system design. As can be seen from the data of Dunning et al., supra, increasing the hydrolysis reaction time from 6 to 15 minutes results in only a 4 percent degradation of glucose.
It has been established that the hydrolysis of cellulose is, in part, limited by the accessibility of the cellulose to the acid (Wenzel, Chemical Technology of Wood). By combining the high shear and mixing potential of today's twin screw extruders, such as done in the dilute acid hydrolysis tests conducted by Rugg et al., supra, with the higher acid concentrations associated with concentrated acid hydrolysis systems, it is possible to deliver the acid necessary to the lignocellulosic structure that will yield results similar to those obtained by Dunning et al. (Industrial and Engineering Chemistry and '586), but in a simpler, more compact, more controllable, and much more economical process.
To emphasize the importance this intensive mixing plays in the impregnation process, Dunning et al., '586, column 5, line 24 through column 6, line 32, noted that when the cellulose rich residue from the hemicellulose (pentosan) hydrolysis step was dried to 50 percent moisture and combined with 85 percent sulfuric acid in a ratio of 0.53 parts acid to one part residue a conversion of only 3 percent glucose was obtained. The same residue when dried to a powder and mixed with the same amount of acid under conditions of "good agitation" then produced a conversion of 60.6 percent glucose. When this same dried residue was mixed with the same amount of acid and subjected to a two-minute pressure treatment, a conversion of 89 percent was achieved.